Discontinuous metal and cermet film resistors and strain gauges

ABSTRACT

Improved discontinuous and cermet film resistors and strain gauges are disclosed wherein the film is maintained in a constant humidity in the range of 15% to 45% thereby substantially increasing the stable life of the strain gauge.

This application is a continuation-in-part of our copending applicationSer. No. 078,486 filed Sept. 24, 1979, U.S. Pat. No. 4,231,011.

The invention relates to discontinuous metal film and cermet filmresistors and strain gauges and particularly to stabilized discontinuousmetal film and cermet film resistors and strain gauges which resist thedeterioration commonly associated with such films.

Discontinuous metal film strain gauges have gauge factors on the orderof 100 to 200. This high level of sensitivity coupled with the potentialfor miniaturization makes them of great interest. Unfortunately, theprincipal problem with such strain gauges is their instability withtime. Over time there occurs an increase in resistance which has beenbelieved to be due to island coalescence or agglomeration. Many attemptshave been made, in the past, to control this problem and moreparticularly to control island coalescence or agglomeration in an effortto stabilize such film resistors and strain gauges.

Studies of the properties and problems of discontinuous metal films andcermet films have proceeded for more than two decades. Electronmicroscope studies reveal the fact that discontinuous metal films aremade up of individual metal grains roughly 100 A in diameter separatedby distances of a few A. The conduction of current in these films isascribed to quantum mechanical tunnelling of the electrons betweengrains. This process, which gives the film high sheet resistance (from afew to many MΩ's), also explains their high sensitivity. The gaugefactor, α, which is defined as the ratio of the fractional resistancechange to the fractional length change or strain (α=ΔR/R/Δl/l), is, asmentioned above, about 100 to 200. In contrast, for ordinary thin film,wire, or foil strain gauges α≈1 to 5. Over a period of time the grainstend to coalesce or aggregate, increasing the average distance betweengrains resulting in an increase in resistance.

Cermet film several hundreds Angstroms thick exhibit gage factors on theorder of 12-16. Thinner cermet film would exhibit higher gage factors,but would suffer instabilities approaching those of discontinuous thinfilm. Over a period of time the grains tend to coalesce or aggregate,increasing the average distance between grains resulting in an increasein resistance.

We have discovered that this problem of coalescence or aggregation canbe controlled by controlling the humidity of the environment in whichthe film is held. We have also found that this can be further aided byirradiation of the substrate surface on which the metal film is applied.However, whether the substrate surface is modified or not, humidity isthe most important parameter determining long term stability and it isessential that the humidity of the environment for the discontinuousfilm be controlled. We have found the control of humidity is bestaccomplished by encapsulating the discontinuous film or cermet filmwithin the desired humidity.

The mechanism by which humidity stabilizes these gauges is not known tous. It may be any of a number of mechanisms, however, it is clear thathumidity control is essential to longevity. We have found that the lifeof the strain gauge is directly related to the humidity of theatmosphere in which it is contained. Gauges stored in a dessicatordeveloped resistances too high to measure with a wheatstone bridge in amatter of one to two weeks. On the other hand gauges stored in 100%humidity showed a drop in resistance from an initial value of about 5 MΩto a final value of about 50 KΩ accompanied by a drop in gauge factorfrom 80-100 to 1-10. Gauges stored in ordinary air having 28% humiditystabilize to easily measurable resistance values and desirable gaugefactors of about 40-110 after some initial variation. Although the gaugeperformed well at 28% humidity, similar performance can be expected ifthe resistor strain gauge is maintained in an environment havinghumidity within the range from 15 % to 45%.

Other details, objects and advantages of the invention will becomeapparent as a present preferred embodiment of the invention proceeds.

In the accompanying drawings, we have shown a present preferredembodiment of the invention in which:

FIG. 1 is a cross-sectional view of a preferred embodiment of theinvention, and

FIG. 2 is a plan view of the embodiment of FIG. 1 wherein part of theoverlay is cut away to show the discontinuous metal film and electrode.

Referring to the drawings a nonconductive substrate 10 is provided onwhich a discontinuous metal film or cermet film 12 has been deposited.Electrodes 14 are attached to the film 12 and wires 16 are connected tothe electrodes. The metal film 12 is encapsulated by an overlay 18 in amanner so as to enclose the film in a moisture proof environment 20thereby maintaining it at a constant predetermined humidity. The metalfilm may be platinum, gold or other suitable metal. The overlay may be arubberlike elastomer, hard plastic or other flexible material having lowpermeability for water vapor. It may in some instances be desirable touse the same material for the overlay 18 and the substrate 10. Whenattaching the overlay 18 to the substrate we prefer to partially coverthe electrodes 14 thereby permitting easy attachment of the wires 16prior to using the device.

This strain gauge can be used in the conventional manner by gluing it tothe workpiece. Alternatively, the film may be deposited on the workpieceand the electrodes and overlay attached thereto. The strain gauge canalso be used in a bridge configuration for temperature compensation.

We prefer to choose overlay material having sufficient flexibility so itwill not affect the gauge's measurements. Nevertheless, one shouldcalibrate the gauge after it has been completed to assure that theoverlay is not affecting the results.

Other means, besides encapsulation can be used to provide a controlledhumidity about the strain gauge. In some applications it may be possibleto place a vapor impermeable cover over the gauge and the workpiece.Whenever covers are used they must be designed and positioned so as notto affect the gauge's readings. If the workpiece and gauge can belocated in a controlled environment such as a laboratory chamber, thegauge need not be encapsulated provided the humidity in the chamber iskept within the prescribed limits.

We have also found that the lifetime of the resistance strain gauge maybe further increased by irradiating the portion of the substrate onwhich the film is deposited. Using a silicon beam we bombarded a glassslide on which the gauge was deposited to produce a displacement ofabout one surface atom per thousand surface atoms. This increased thelifetime of the gauge by three to seven days depending upon theirradiation levels. An irradiation level of DPA=0.73×10⁻³ increased thelifetime by about three days. A similar increase was experienced for aDPA of 1.8×10⁻³. At a DPA=2.4×10⁻³ and DPA=4×10⁻³ the lifetime wasincreased by about a week.

While we have shown and described a present preferred embodiment of theinvention it is to be distinctly understood that the invention is notlimited thereto but may be otherwise variously embodied within the scopeof the following claims.

We claim:
 1. A strain gauge comprising:a. a substrate composed ofnonconductive material, b. a cermet film on the substrate, c. aplurality of electrodes attached to the cermet film, and d. means formaintaining an environment having a constant humidity in the range of15% to 45% about the discontinuous metal film.
 2. The strain gauge ofclaim 1 wherein the means for maintaining the constant humidityenvironment is encapsulation.
 3. The strain gauge of claim 2 wherein thegauge is encapsulated by a flexible material.
 4. The strain gauge ofclaim 1 wherein the humidity is 28%.
 5. The strain gauge of claim 1wherein the substrate has been irradiated.